The present invention relates to DC current testing of integrated circuits and, more particularly, to a computer-based test apparatus and method.
After an integrated circuit is fabricated, several tests are performed on the integrated circuit to verify its functionality and performance. One of the tests commonly performed on an integrated circuit is a DC current test. During DC current testing, the integrated circuit is coupled to a tester which applies a voltage across the power supply pads of the integrated circuit and then measures the DC current drawn through the power supply pads. The integrated circuit is typically conditioned before making the current measurement by applying a set of test vectors to the input-output (IO) pads of the integrated circuit. These test vectors place the integrated circuit in a quiescent mode in which all sources of DC current in the integrated circuit are turned off. If the measured current exceeds a specified tolerance, there may be a short or some other fault condition in the integrated circuit. This test is typically run as a pass/fail test. Integrated circuits that fail the test are labeled to indicate the failure. Integrated circuits that pass the test are evaluated further with other test procedures.
DC current testing is typically automated to some degree by using a computer-based test apparatus. The test sequence and test vectors are contained in a software program that is run by the tester. The test engineer loads the test program into local memory and then enters an expected measurement range for the tester. This measurement range reflects the current level that the test engineer expects to be drawn by the integrated circuit being tested. For example, if the engineer expects that the integrated circuit will draw about 35 .mu.A, the test engineer may select a measurement range of 0-100 .mu.A. The test engineer then compiles the test program with the selected measurement range on the tester, and initiates the test sequence. The tester measures the DC current and provides an output indicative of whether the measured DC current is above or below the tolerance level. For example, the test apparatus may light a green light or a red light to indicate whether the integrated circuit has passed or failed the test. The integrated circuit is then removed and replaced with the next integrated circuit to be tested.
This test procedure is relatively accurate and efficient for a simple pass/fail test but is relatively inaccurate and inefficient for obtaining the actual value of the current that is drawn by the integrated circuit. For example, if the integrated circuit draws 5 mA instead of 35 .mu.A and the tester is set to the 0-100 .mu.A range, the tester may simply output the maximum current in the range or may generate some other "out of range" signal. The test engineer must then modify the test program to increase the measurement range, recompile the test program and re-run the test sequence. This process may need to be repeated several times until the correct measurement range is selected. As a result, this test procedure is very time consuming and inefficient if a large number of integrated circuits are tested and precise current measurements are required.